To secure communications data and stored data, it is becoming increasingly prevalent to use the cryptographic technique of encryption. In this context, the data are enciphered under the control of a cryptographic key. The data can also be deciphered again using the same key. Marketable products and software libraries are available for this purpose.
In encryption operations, a so-called hybrid method is mostly used. In this method, the actual message is encrypted using a randomly selected symmetric key (session key) and a preselected symmetric encryption method (e.g., Data Encryption Standard (DES), International Data Encryption Algorithm (IDEA)). The session key is then encrypted, in turn, in each case using the public key of the receiver (a plurality of receivers is possible) and using a predefined asymmetric or public key method (e.g., Rivest, Shamir, Aldeman Code(RSA), EIGamal (a public key encryption algorithm)). The session key encrypted using this process is included with the encrypted message for each receiver. A description of this procedure and of the algorithms employed is found, for example, in William Stallings: “Cryptography and Network Security: Principles and Practice”, Prentice Hall, Upper Saddle River, N.J., 1998.
To decode a message, the receiver must first decipher the session key using his private key, which belongs to his public key, and a preselected public key algorithm, to then decrypt the message using this session key.
Besides encrypting messages, cryptographic methods are also used to encrypt stored data, e.g., on one's own personal computer. Here as well, one typically employs a hybrid method, where the user first encrypts the data using a randomly selected symmetric key (session key) and a predefined symmetric encryption method (e.g., DES, IDEA). The session key is then encrypted, in turn, using the user's public key and a preselected asymmetric or public key method (e.g., RSA, EIGamal).
Using his or her private key, which belongs to his or her public key, and a predefined public key algorithm, the user first encrypts the session key and then, a using this session key, the stored data.
In the following, the term “encryption key” is used in each case to refer to the private key of the user, i.e., of the receiver.
The encryption key is either stored on a smart card, access to the smart card being protected by a personal identification number (PIN) known only to the user, or it is stored on another storage medium (for example a hard disk or diskette), in which case it is protected preferably by a long password.
It can happen that the encryption key is lost. For example, if the storage medium where it was located is destroyed, or if the user forgets the PIN number or the password which he or she used to secure the encryption key, then it is no longer possible to use it to access the encrypted data.
To be able to make encrypted data accessible again in the event the encryption key is lost, mechanisms are needed to enable the encryption key to be regenerated in a secure manner. For this purpose, the encryption key is typically generated nowadays at a trust center or a confidential central location and securely stored. As a rule, the encryption key is produced by initially generating a large random number (seed) using a statistically valid random process. From this random number, the key pair made up of the public key/private key is then generated with the aid of a deterministic method. This seed is subsequently deleted. If necessary, a copy of his or her encryption key is then delivered to the user for use.
In the process, the user does not have any influence on how his or her encryption key is generated and further handled by the generator. It is expensive to securely transport the copy of the encryption key to the user. As a transport medium, the smart card mentioned above can be used, for example, which is sent to the user.